Cursor Auth: Windows login opens cmd window but does not launch browser

### Repro
1. Open Cursor provider login on Windows.
2. A cmd window pops up.
3. Browser login page does not open, or flow appears stuck.

### Expected
Cursor login URL should open in the default browser.

### Actual
Only a cmd window appears.

### Root cause
`openBrowser()` uses `start ""` on win32, which treats the first quoted arg as a window title on Windows.

### Suggested fix
Use:
```
if (platform === 'win32') {
exec(`cmd /c start "" "${url}"`);
return;
}
```

This makes the URL actually open in the browser.

Start at the openBrowser() entry point and inspect its win32 handling, then reproduce the Cursor provider login flow on Windows. Done means the login URL opens in the default browser without leaving the flow stuck or exposing an unwanted cmd window.

Written by the indexing model from the issue text.